Variable speed compressor protection system and method

ABSTRACT

A system and method for a compressor includes a compressor connected to a condenser, a discharge line temperature sensor that outputs a discharge line temperature signal corresponding to a discharge line temperature of refrigerant leaving the compressor, and a control module connected to the discharge line temperature sensor. The control module determines a saturated condenser temperature, calculates a discharge superheat temperature based on the saturated condenser temperature and the discharge line temperature, and monitors a flood back condition of the compressor by comparing the discharge superheat temperature with a predetermined threshold. The control module increases a speed of the compressor or decreases an opening of an expansion valve associated with the compressor when the discharge superheat temperature is less than or equal to the predetermined threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 12/246,959 filed on Oct. 7, 2008. This application claims the benefit of U.S. Provisional Application No. 60/978,258, filed on Oct. 8, 2007. The entire disclosures of each of the above applications are incorporated herein by reference.

FIELD

The present disclosure relates to compressors, and more particularly, to a protection system for use with a variable speed compressor.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Compressors may be used in a wide variety of industrial and residential applications to circulate refrigerant within a refrigeration, heat pump, HVAC, or chiller system (generically “refrigeration systems”) to provide a desired heating or cooling effect. In any of the foregoing applications, the compressor should provide consistent and efficient operation to insure that the particular application (i.e., refrigeration, heat pump, HVAC, or chiller system) functions properly. A variable speed compressor may be used to vary compressor capacity according to refrigeration system load.

Operation of the compressor during a flood back condition is undesirable. A flood back condition occurs when excessive liquid refrigerant flows into the compressor. Severe flood back can dilute the oil and reduce its lubrication property, leading to potential seizure. Although some mixture of liquid refrigerant and oil in the compressor may be expected, excessive mixture may cause damage to the compressor.

Likewise, operation of the compressor at excessive temperature levels may be damaging to the compressor. An overheat condition may damage internal compressor components including, for example, the electric motor.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

A system is provided that includes a compressor connected to a condenser and a discharge line temperature sensor that outputs a discharge line temperature signal corresponding to a discharge line temperature of refrigerant leaving the compressor. The system also includes a control module connected to the discharge line temperature sensor. The control module determines a saturated condenser temperature, calculates a discharge superheat temperature based on the saturated condenser temperature and the discharge line temperature, and monitors a flood back condition of the compressor by comparing the discharge superheat temperature with a predetermined threshold. The control module also increases a speed of the compressor or decreases an opening of an expansion valve associated with the compressor when the discharge superheat temperature is less than or equal to the predetermined threshold.

A method is also provided and includes determining, with a control module, a saturated condenser temperature of a condenser connected to a compressor. The method also includes receiving, with the control module, a discharge line temperature signal that corresponds to a discharge line temperature of refrigerant leaving the compressor. The method also includes calculating, with the control module, a discharge superheat temperature based on the saturated condenser temperature and the discharge line temperature. The method also includes monitoring, with the control module, a flood back condition of the compressor by comparing the discharge superheat temperature with a predetermined threshold. The method also includes increasing a speed of the compressor or decreasing an opening of an expansion valve associated with the compressor when the discharge superheat temperature is less than or equal to the predetermined threshold.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a schematic illustration of a refrigeration system.

FIG. 2 is a perspective view of a compressor with an inverter drive.

FIG. 3 is another perspective view of a compressor with an inverter driver.

FIG. 4 is a cross-section view of a compressor.

FIG. 5 is a graph showing discharge super heat correlated with suction super heat and outdoor temperature.

FIG. 6 is a graph showing condenser temperature correlated with compressor power and compressor speed.

FIG. 7 is a graph showing an operating envelope of a compressor.

FIG. 8 is a graph showing condensing temperature correlated with evaporator temperature and compressor power for a given compressor speed.

FIG. 9 is a graph showing discharge line temperature correlated with evaporator temperature and condenser temperature.

FIG. 10 is a flow chart showing derived data for a refrigeration system.

FIG. 11 is a schematic of a refrigeration system.

FIG. 12 is a flow chart showing derived data for a refrigeration system.

FIG. 13 is a graph showing mass flow correlated with inverter drive heat loss.

FIG. 14 is a graph showing inverter speed correlated with inverter efficiency.

FIG. 15 is a graph showing a control module with measured inputs and derived outputs.

FIG. 16 is a schematic of a refrigeration system.

FIG. 17 is a cross-section view of a compressor.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

As used herein, the terms module, control module, and controller may refer to one or more of the following: An application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. As used herein, computer readable medium may refer to any medium capable of storing data for a computer or module, including a processor. Computer-readable medium includes, but is not limited to, memory, RAM, ROM, PROM, EPROM, EEPROM, flash memory, CD-ROM, floppy disk, magnetic tape, other magnetic medium, optical medium, or any other device or medium capable of storing data for a computer.

With reference to FIG. 1, an exemplary refrigeration system 5 includes a compressor 10 that compresses refrigerant vapor. While a specific refrigeration system is shown in FIG. 1, the present teachings are applicable to any refrigeration system, including heat pump, HVAC, and chiller systems. Refrigerant vapor from compressor 10 is delivered to a condenser 12 where the refrigerant vapor is liquefied at high pressure, thereby rejecting heat to the outside air. The liquid refrigerant exiting condenser 12 is delivered to an evaporator 16 through an expansion valve 14. Expansion valve 14 may be a mechanical or electronic valve for controlling super heat of the refrigerant. The refrigerant passes through expansion valve 14 where a pressure drop causes the high pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. As hot air moves across evaporator 16, the low pressure liquid turns into gas, thereby removing heat from evaporator 16. The low pressure gas is again delivered to compressor 10 where it is compressed to a high pressure gas, and delivered to condenser 12 to start the refrigeration cycle again.

With reference to FIGS. 1, 2 and 3, compressor 10 may be driven by an inverter drive 22, also referred to as a variable frequency drive (VFD), housed in an enclosure 20. Enclosure 20 may be near compressor 10. Inverter drive 22 receives electrical power from a power supply 18 and delivers electrical power to compressor 10. Inverter drive 22 includes a control module 25 with a processor and software operable to modulate and control the frequency of electrical power delivered to an electric motor of compressor 10. Control module 25 includes a computer readable medium for storing data including the software executed by the processor to modulate and control the frequency of electrical power delivered to the electric motor of compressor and the software necessary for control module 25 to execute and perform the protection and control algorithms of the present teachings. By modulating the frequency of electrical power delivered to the electric motor of compressor 10, control module 25 may thereby modulate and control the speed, and consequently the capacity, of compressor 10.

Inverter drive 22 includes solid state electronics to modulate the frequency of electrical power. Generally, inverter drive 22 converts the inputted electrical power from AC to DC, and then converts the electrical power from DC back to AC at a desired frequency. For example, inverter drive 22 may directly rectify electrical power with a full-wave rectifier bridge. Inverter driver 22 may then chop the electrical power using insulated gate bipolar transistors (IGBT's) or thyristors to achieve the desired frequency. Other suitable electronic components may be used to modulate the frequency of electrical power from power supply 18.

Electric motor speed of compressor 10 is controlled by the frequency of electrical power received from inverter driver 22. For example, when compressor 10 is driven at sixty hertz electric power, compressor 10 may operate at full capacity operation. When compressor 10 is driven at thirty hertz electric power, compressor 10 may operate at half capacity operation.

Piping from evaporator 16 to compressor 10 may be routed through enclosure 20 to cool the electronic components of inverter drive 22 within enclosure 20. Enclosure 20 may include a cold plate 15. Suction gas refrigerant may cool the cold plate prior to entering compressor 10 and thereby cool the electrical components of inverter drive 22. In this way, cold plate 15 may function as a heat exchanger between suction gas and inverter drive 22 such that heat from inverter drive 22 is transferred to suction gas prior to the suction gas entering compressor 10.

As shown in FIGS. 2 and 3, electric power from inverter drive 22 housed within enclosure 20 may be delivered to compressor 10 via a terminal box 24 attached to compressor 10.

A compressor floodback or overheat condition is undesirable and may cause damage to compressor 10 or other refrigeration system components. Suction super heat (SSH) and/or discharge super heat (DSH) may be correlated to a flood back or overheating condition of compressor 10 and may be monitored to detect and/or predict a flood back or overheating condition of compressor 10. DSH is the difference between the temperature of refrigerant vapor leaving the compressor, referred to as discharge line temperature (DLT) and the saturated condenser temperature (Tcond). Suction super heat (SSH) is the difference between the temperature of refrigerant vapor entering the compressor, referred to as suction line temperature (SLT) and saturated evaporator temperature (Tevap).

SSH and DSH may be correlated as shown in FIG. 5. The correlation between DSH and SSH may be particularly accurate for scroll type compressors, with outside ambient temperature being only a secondary effect. As shown in FIG. 5, correlations between DSH and SSH are shown for outdoor temperatures (ODT) of one-hundred fifteen degrees Fahrenheit, ninety-five degrees Fahrenheit, seventy-five degrees Fahrenheit, and fifty-five degrees Fahrenheit. The correlation shown in FIG. 5 is an example only and specific correlations for specific compressors may vary by compressor type, model, capacity, etc.

A flood back condition may occur when SSH is approaching zero degrees or when DSH is approaching twenty to forty degrees Fahrenheit. For this reason, DSH may be used to detect the onset of a flood back condition and its severity. When SSH is at zero degrees, SSH may not indicate the severity of the flood back condition. As the floodback condition becomes more severe, SSH remains at around zero degrees. When SSH is at zero degrees, however, DSH may be between twenty and forty degrees Fahrenheit and may more accurately indicate the severity of a flood back condition. When DSH is in the range of thirty degrees Fahrenheit to eighty degrees Fahrenheit, compressor 10 may operate within a normal range. When DSH is below thirty degrees Fahrenheit, the onset of a flood back condition may occur. When DSH is below ten degrees Fahrenheit, a severe flood back condition may occur.

With respect to overheating, when DSH is greater than eighty degrees Fahrenheit, the onset of an overheating condition may occur. When DSH is greater than one-hundred degrees Fahrenheit, a severe overheating condition may be present.

In FIG. 5, typical SSH temperatures for exemplar refrigerant charge levels are shown. For example, as the percentage of refrigerant charge in refrigeration system 5 decreases, SSH typically increases.

To determine DSH, DLT may be subtracted from Tcond. DLT may be sensed by a DLT sensor 28 that senses a temperature of refrigerant exiting compressor 10. As shown in FIG. 1, DLT sensor 28 may be external to compressor 10 and may be mounted proximate a discharge outlet of compressor 10. Alternatively, an internal DLT sensor 30 may be used as shown in FIG. 4. In FIG. 4, a cross-section of compressor 10 is shown. Internal DLT sensor 30 may be embedded in an upper fixed scroll of a scroll compressor and may sense a temperature of discharge refrigerant exiting the intermeshing scrolls.

In the alternative, a combination temperature/pressure sensor may be used. In such case, Tcond may be measured based on the pressure of refrigerant exiting compressor 10 as measured by the combination sensor. Moreover, in such case, DSH may be calculated based on DLT, as measured by the temperature portion of the sensor, and on Tcond, as measured by the pressure portion of the combination sensor.

Tcond may be derived from other system parameters. Specifically, Tcond may be derived from compressor current and voltage (i.e., compressor power), compressor speed, and compressor map data associated with compressor 10. A method for deriving Tcond based on current, voltage and compressor map data for a fixed speed compressor is described in the commonly assigned application for Compressor Diagnostic and Protection System, U.S. application Ser. No. 11/059,646, Publication No. U.S. 2005/0235660. Compressor map data for a fixed speed compressor correlating compressor current and voltage to Tcond may be compressor specific and based on test data for a specific compressor type, model and capacity.

In the case of a variable speed compressor, Tcond may also be a function of compressor speed, in addition to compressor power.

A graphical correlation between compressor power in watts and compressor speed is shown in FIG. 6. As shown, Tcond is a function of compressor power and compressor speed. In this way, a three-dimensional compressor map with data correlating compressor power, compressor speed, and Tcond may be derived for a specific compressor based on test data. Compressor current may be used instead of compressor power. Compressor power, however, may be preferred over compressor current to reduce the impact of any line voltage variation. The compressor map may be stored in a computer readable medium accessible to control module 25.

In this way, control module 25 may calculate Tcond based on compressor power data and compressor speed data. Control module 25 may calculate, monitor, or detect compressor power data during the calculations performed to convert electrical power from power supply 18 to electrical power at a desired frequency. In this way, compressor power and current data may be readily available to control module 25. In addition, control module 25 may calculate, monitor, or detect compressor speed based on the frequency of electrical power delivered to the electric motor of compressor 10. In this way, compressor speed data may also be readily available to control module 25. Based on compressor power and compressor speed, control module 25 may derive Tcond.

After measuring or calculating Tcond, control module 25 may calculate DSH as the difference between Tcond and DLT, with DLT data being receiving from external DLT sensor 28 or internal DLT sensor 30.

Control module 25 may monitor DSH to detect a flood back or overheat condition, based on the correlation between DSH and flood back and overheat conditions described above. Upon detection of a flood back or overheat condition, control module 25 may adjust compressor speed or adjust expansion valve 14 accordingly. Control module 25 may communicate with or control expansion valve 14. Alternatively, control module 25 may communicate with a system controller for refrigeration system 5 and may notify system controller of the flood back or overheat condition. System controller may then adjust expansion valve or compressor speed accordingly.

DSH may be monitored to detect or predict a sudden flood back or overheat condition. A sudden reduction in DLT or DSH without significant accompanying change in Tcond may be indicative of a sudden flood back or overheat condition. For example, if DLT or DSH decreases by a predetermined temperature amount (e.g., fifty degrees Fahrenheit) within a predetermined time period (e.g., fifty seconds), a sudden flood back condition may exist. Such a condition may be caused by expansion valve 14 being stuck open. Likewise, a sudden increase in DLT or DSH with similar magnitude and without significant accompanying change in Tcond may be indicative of a sudden overheat condition due to expansion valve 14 being stuck closed. For example, if DLT or DSH increases by a predetermined temperature amount (e.g., fifty degrees Fahrenheit) within a predetermined time period (e.g., fifty seconds), a sudden overheat condition may exist.

Control module 25 may monitor DSH and DLT to determine whether compressor 10 is operating within a predetermined operating envelope. As shown in FIG. 7, a compressor operating envelope may provide maximum flood back and maximum and/or minimum DSH/DLT limits. In addition, a maximum scroll temperature limit (Tscroll) may be provided, in the case of a scroll compressor. In addition, a maximum motor temperature (Tmotor) may be provided. As shown in FIG. 7, compressor speed and expansion valve 14 may be adjusted based on DSH and/or DLT to insure compressor operation within the compressor operating envelope. In this way, DSH and/or DLT may move back into an acceptable range as indicated by FIG. 7. Compressor speed adjustment may take priority over expansion valve adjustment. In some cases, such as a defrost state, it may be difficult for expansion valve 14 to respond quickly and compressor speed adjustment may be more appropriate.

In the event of a flood back condition, control module 25 may limit a compressor speed range. For example, when DSH is below thirty degrees Fahrenheit, compressor operation may be limited to the compressor's cooling capacity rating speed. For example, the cooling capacity rating speed may be 4500 RPM. When DSH is between thirty degrees Fahrenheit and sixty degrees Fahrenheit, compressor operating speed range may be expanded linearly to the full operating speed range. For example, compressor operating speed range may be between 1800 and 7000 RPM.

The function correlating Tcond with compressor speed and power, may assume a predetermined or constant saturated Tevap. As shown in FIG. 8, the correlation between compressor power and Tcond may be insensitive to variations of Tevap.

For additional accuracy, Tevap may be derived as a function of Tcond and DLT, as described in commonly assigned U.S. application Ser. No. 11/059,646, U.S. Publication No. 2005/0235660. For variable speed compressors, the correlation may also reflect compressor speed. In this way, Tevap may be derived as a function of Tcond, DLT and compressor speed.

As shown in FIG. 9, Tevap is shown correlated with DLT, for various Tcond levels. For this reason, compressor map data for different speeds may be used.

Tcond and Tevap may be calculated based on a single derivation.

In addition, iterative calculations may be made based on the following equations: Tcond=f(compressor power,compressor speed,Tevap)  Equation 1: Tevap=f(Tcond,DLT,compressor speed)  Equation 2:

Multiple iterations of these equations may be performed to achieve convergence. For example, three iterations may provide optimal convergence. As discussed above, more or less iteration, or no iterations, may be used.

Tevap and Tcond may also be determined by using compressor map data, for different speeds, based on DLT and compressor power, based on the following equations: Tevap=f(compressor power,compressor speed,DLT)  Equation 3: Tcond=f(compressor power,compressor speed,DLT)  Equation 4:

Once Tevap and Tcond are known, additional compressor performance parameters may be derived. For example, compressor capacity and compressor efficiency may be derived based on additional compressor performance map data for a specific compressor model and capacity. Such additional compressor map data may be derived from test data. For example, compressor mass flow or capacity, may be derived according to the following equation: Tevap=f(compressor speed,Tcond,mass flow)  Equation 5:

Mass flow may be derived according to the following equation: Mass Flow=m0+m1*Tevap+m2*Tcond+m3*RPM+m4*Tevap*Tcond+m5*Tevap*RPM+m6*Tcond*RPM+m7*Tevap^2+m8*Tcond^2+m9*RPM^2+m10*Tevap*Tcond*RPM+m11*Tevap^2*Tcond+m12*Tevap^2*RPM+m13*Tevap^3+m14*Tevap*Tcond^2+m15*Tcond^2*RPM+m16*Tcond^3+m17*Tevap*RPM^2+m18*Tcond*RPM^2+m19*RPM^3  Equation 6:

where m0-m19 are compressor model and size specific, as published by compressor manufacturers.

Compressor map data may be stored within a computer readable medium within control module 25 or accessible to control module 25.

As shown in FIG. 10, a flow chart for derived parameters is shown. In step 100, Tcond may be derived from compressor power and compressor speed. In step 101, Tevap may be derived from DLT and Tcond. In step 102, capacity/mass flow and a compressor energy efficiency ratio may be derived from Tevap and Tcond. In addition, by monitoring run time in step 103, additional parameters may be derived. Specifically, in step 104, load and Kwh/Day may be derived from run time, capacity/mass flow, EER, and compressor power.

By monitoring the above operating parameters, control module 25 may insure that compressor 10 is operating within acceptable operating envelope limits that are preset by a particular compressor designer or manufacturer and may detect and predict certain undesirable operating conditions, such as compressor floodback and overheat conditions. Further, control module 25 may derive other useful data related to compressor efficiency, power consumption, etc.

Where compressor 10 is driven by a suction cooled inverter drive 22, Tevap may be alternatively calculated. Because Tevap may be calculated from mass flow, Tcond, and compressor speed as discussed above, control module 25 may derive mass flow from a difference in temperature between suction gas entering cold plate 15 (Ts) and a temperature of a heat sink (Ti) located on or near inverter drive 22. Control module 25 may calculate delta T according to the following equation: delta T=Ts−Ti  Equation 7:

Ts and Ti may be measured by two temperature sensors 33 and 34 shown in FIG. 11. Temperature sensor 33 measures the temperature of the heat sink (Ti) and may be incorporated as part of inverter drive 22. Alternatively, temperature sensor 33 may measure a temperature of suction gas exiting cold plate 15 and may be located on or near the piping between cold plate 15 and compressor 10. Temperature sensor 34 measures the temperature of suction gas entering cold plate 15.

Control module 25 may determine mass flow based on delta T and by determining the applied heat of inverter drive 22. As shown in FIG. 12, mass flow may be derived based on lost heat of inverter drive 22 and delta T. As shown in FIG. 13, the relationship between mass flow, delta T and applied inverter heat may be mapped based on test data.

Inverter heat may be derived based on inverter speed (i.e., compressor speed) and inverter efficiency as shown in FIG. 14.

With reference again to FIG. 12, inputs include compressor speed (RPM) 120, compressor current 122, compressor voltage 124, compressor power factor 126, Ti 128 and Ts 130. From compressor current 122, compressor voltage 124, and power factor 126, compressor power 132 is derived. From temperatures Ti 128 and Ts 130, delta T 134 is derived. From RPM 120 and power, Tcond 136 is derived. Also from RPM 120 and power 132, inverter heat loss 138 is derived. From inverter heat loss, and delta T 134, mass flow 140 is derived. From RPM 120, Tcond 136, and mass flow 140, Tevap 142 is derived. From Tevap 142 and Ts 130, SSH 144 is derived. From SSH 144 and ambient temperature as sensed by ambient temperature sensor 29, DSH 146 is derived. Once DSH 146 is derived, all of the benefits of the algorithms described above may be gained, including protection of compressor 10 from flood back and overheat conditions.

As shown by dotted line 141, Tcond and Tevap may be iteratively calculated to more accurately derive Tcond and Tevap. For example, optimal convergence may be achieved with three iterations. More or less iterations may also be used.

As shown in FIG. 15, control module 25 takes as measured inputs compressor speed RPM, inverter drive current, voltage, and power, and heat sink temperatures Ti and Ts. Control module also takes as input ambient temperature as indicated by ambient temperature sensor 29. As discussed above, control module 25 derives from these measured inputs the outputs of Tcond, Tevap, mass flow, SSH, DSH, and DLT.

As shown in FIG. 16, control module 25 may monitor SLT with SLT sensor 35, which may include a combination pressure and temperature sensor may be used. In such case, Tevap may be measured based on the suction pressure as measured by the pressure portion of the combination sensor. Further, SSH may be calculated based on SLT, as measured by the temperature portion of the combination sensor, and Tevap. SLT sensor 34, 35 may be located at an inlet to compressor 10 and may sense a temperature or pressure of refrigerant entering compressor 10 subsequent to inverter 22, enclosure 20, or cold plate 15. Alternatively SLT sensor may be located at an inlet to enclosure 20, inverter 22, or cold plate 15 and may sense a temperature or pressure of refrigerant entering the enclosure 20, inverter 22, or cold plate 15.

In addition, similar to the calculation of DSH based on DLT described above, control module 25 may also calculate SSH. For example, compressor power, compressor speed, and compressor map data may be used to derive Tcond and Tevap may be derived from Tcond. Once Tevap is derived, SSH may be derived from SLT and Tevap and used as described above for monitoring various compressor operating parameters and protecting against flood back and overheat conditions. 

What is claimed is:
 1. A system comprising: a compressor connected to a condenser; a discharge line temperature sensor that outputs a discharge line temperature signal corresponding to a discharge line temperature of refrigerant leaving the compressor; a control module connected to the discharge line temperature sensor, said control module receiving compressor power data, determining a saturated condenser temperature as a function of the compressor power data and a speed of the compressor, calculating a discharge superheat temperature based on the saturated condenser temperature and the discharge line temperature, monitoring a flood back condition of the compressor by comparing the discharge superheat temperature with a predetermined threshold, and, when the discharge superheat temperature is less than or equal to the predetermined threshold, increasing the speed of the compressor or decreasing an opening of an expansion valve associated with the compressor.
 2. The system of claim 1 wherein the predetermined threshold is thirty degrees Fahrenheit.
 3. The system of claim 1 wherein the control module increases the speed of the compressor when the discharge superheat temperature is less than or equal to the predetermined threshold.
 4. The system of claim 1 wherein the control module decreases the opening of the expansion valve when the discharge superheat temperature is less than or equal to the predetermined threshold.
 5. The system of claim 1 wherein the control module monitors a sudden flood back condition by determining whether the discharge superheat temperature decreases by a predetermined amount within a predetermined time period.
 6. The system of claim 1 wherein the control module receives a discharge pressure signal corresponding to a discharge pressure of refrigerant leaving the compressor and determines the saturated condenser temperature based on the discharge pressure.
 7. The system of claim 1 further comprising an evaporator connected to the compressor and the condenser, wherein the control module receives compressor power data, determines a saturated evaporator temperature as a function of the saturated condenser temperature, the discharge line temperature, and the speed of the compressor, and determines the saturated condenser temperature as a function of the compressor power data, the speed of the compressor, and the saturated evaporator temperature.
 8. The system of claim 7 wherein the control module performs multiple iterations of determining the saturated condenser temperature and the saturated evaporator temperature to achieve convergence.
 9. The system of claim 1 wherein the control module receives compressor power data and determines the saturated condenser temperature as a function of the compressor power data, the speed of the compressor, and the discharge line temperature.
 10. A method comprising: receiving, with a control module, compressor power data of a compressor; determining, with the control module, a saturated condenser temperature of a condenser connected to the compressor as a function of the compressor power data and a speed of the compressor; receiving, with the control module, a discharge line temperature signal that corresponds to a discharge line temperature of refrigerant leaving the compressor; calculating, with the control module, a discharge superheat temperature based on the saturated condenser temperature and the discharge line temperature; monitoring, with the control module, a flood back condition of the compressor by comparing the discharge superheat temperature with a predetermined threshold; and increasing the speed of the compressor or decreasing an opening of an expansion valve, with the control module, when the discharge superheat temperature is less than or equal to the predetermined threshold.
 11. The method of claim 10 wherein the predetermined threshold is thirty degrees Fahrenheit.
 12. The method of claim 10 wherein the control module increases the speed of the compressor when the discharge superheat temperature is less than or equal to the predetermined threshold.
 13. The method of claim 10 wherein the control module decreases the opening of the expansion valve when the discharge superheat temperature is less than or equal to the predetermined threshold.
 14. The method of claim 10 further comprising monitoring, with the control module, a sudden flood back condition by determining whether the discharge superheat temperature decreases by a predetermined amount within a predetermined time period.
 15. The method of claim 10 further comprising receiving, with the control module, a discharge pressure signal corresponding to a discharge pressure of refrigerant leaving the compressor and determining the saturated condenser temperature based on the discharge pressure.
 16. The method of claim 10 further comprising receiving, with the control module, compressor power data, determining a saturated evaporator temperature of an evaporator connected to the compressor and the condenser as a function of the saturated condenser temperature, the discharge line temperature, and the speed of the compressor, and determining the saturated condenser temperature as a function of the compressor power data, the speed of the compressor, and the saturated evaporator temperature.
 17. The method of claim 16 further comprising performing, with the control module, multiple iterations of determining the saturated condenser temperature and the saturated evaporator temperature to achieve convergence.
 18. The method of claim 10 further comprising receiving, with the control module, compressor power data and determining the saturated condenser temperature as a function of the compressor power data, the speed of the compressor, and the discharge line temperature. 